1. Field of the Invention
The present invention relates to a battery control device for controlling charging and discharging operations with respect to battery packs mounted on, for example, a variety of electric vehicles including automated guided vehicles, hybrid electric vehicles and fuel battery electric vehicles, and devices driven by rechargeable batteries, such as uninterruptible power supplies.
2. Description of the Related Art
Conventionally, as a low-pollution vehicle designed for the purpose of solving environmental problems and energy problems, an electric vehicle such as a HEV (hybrid electric vehicle) and a PEV (pure electric vehicle) has received a great deal of attention. The electric vehicle has rechargeable batteries (a battery pack) mounted therein, and the electric power of the battery pack drives an electric motor so as to run the electric vehicle.
The electric vehicle includes an inverter for controlling motor drive and a battery control device for obtaining an output state of the battery pack and controlling charging and discharging operations with respect to the battery pack according to the output state of the battery pack, so as to stably supply the electric power to the electric motor via the inverter.
The battery control device measures a battery voltage, etc., of each battery block (which includes a plurality of combined battery cells) included in the battery pack so as to calculate a remaining capacity SOC (State Of Charge; a value representing how much electric power remains in the rechargeable batteries) of the battery pack based on the measurement result. For example, the battery control device controls the charge and discharge operations with respect to the battery pack so as to keep the remaining capacity SOC of the battery pack within a prescribed range. Further, the battery control device detects abnormality of the battery cell in each battery block based on the battery voltage measured for each battery block.
For example, in Japanese Laid-Open Patent Publication No. 7-137612, entitled xe2x80x9cBattery Diagnostic Devicexe2x80x9d, the battery pack is divided into voltage detection units (battery blocks) each including n battery cells and is monitored by comparing the respective measured voltage values of the voltage detection units with one another so as to detect the abnormal battery cell in the battery pack. Specifically, a voltage of each battery block in the battery pack is measured by a voltage sensor provided in each battery block. When a measured battery block voltage has the same value as the other battery block voltages, the measured battery block voltage is determined to be normal. On the other hand, when the measured battery block voltage has a different value from the other battery block voltages, the measured battery block voltage is determined to be abnormal. For example, in the case where the abnormal battery block includes internal short-circuit battery cells, the detected battery pack voltage is always lower than the other battery pack voltages by a voltage level equivalent to a reduced voltage level of the short-circuit battery cells. Further, in the case where the abnormal battery block includes battery cells in which internal resistance is increased when any abnormality is caused, the level of the detected battery pack voltage is detected to be lower than those of the other battery pack voltages during a discharge operation (during the operation of the battery pack), and the level of the detected battery pack voltage is detected to be higher than those of the other normal battery pack voltages during a regeneration operation (or a charge operation).
Further, in Japanese Laid-Open Patent Publication No. 9-117072, entitled xe2x80x9cProtection Network for Secondary Batteryxe2x80x9d, the respective voltages of voltage detection units (battery blocks) in the battery pack is measured so as to be compared to one another. When the difference in voltage level between the voltage detection units exceeds a prescribed value, it is determined that there is abnormality in the battery pack, so that current flowing into the battery pack is blocked. Specifically, when spot-welded electrical connections in a cell bank are broken, the difference in voltage level between the cell banks is detected, so that a closed circuit connected to the battery pack is electrically disconnected from the battery pack so as to protect the battery pack from being overcharged or overdischarged.
Furthermore, in Japanese Laid-Open Patent Publication No. 11-178225, entitled xe2x80x9cBattery Control Devicexe2x80x9d, the voltage detection units (battery blocks) in the battery pack are grouped according to their temperatures such that voltage detection units having similar temperatures belong to the same group. A maximum voltage difference is calculated for each group. When the calculated maximum voltage difference value of a group is equal to or more than a first threshold, the amount of electric power discharged from that group is limited. Further, when the calculated maximum voltage difference value is equal to or more than a second threshold, the discharge operation is terminated. In this manner, these two steps of controlling the discharge operation are performed according to the state of charge of the battery pack so as to maintain the reliability of the battery pack.
However, in Japanese Laid-Open Patent Publication No. 7-137612, unevenness in battery capacity is not taken into consideration so that the settings of voltage levels are not established in consideration of battery characteristics. Therefore, when the difference in voltage detected between the battery blocks in the battery pack is equal to or more than a prescribed value, the battery pack is immediately and erroneously determined to be abnormal, thereby deteriorating the reliability of the battery pack.
Further, in Japanese Laid-Open Patent Publication No. 9-117072, it is detected that electrical connections in the voltage detection unit are broken based on the difference between the measured voltage values. However, this is merely used for protecting the battery pack from being overcharged and overdischarged so as to maintain the reliability of the battery pack but is not used for detecting the abnormality (overdischarge) of a battery cell in the battery pack.
Furthermore, in Japanese Laid-Open Patent Publication No. 11-178225, the number n of battery cells included in each voltage detection unit (battery block) is required to be sufficient to allow for the distinction between the two-step thresholds. However, unevenness in internal resistance among the battery cells is not taken into consideration, and therefore the number n of the battery cells included in each battery block cannot be sufficiently large. In particular, when the battery pack is used with a high current, there is a possibility that the abnormality of the battery cells might be mistakenly detected, and therefore it is necessary to considerably limit the number of battery cells included in each voltage detection unit. This deteriorates the reliability of the battery pack.
In the above-described conventional technologies, the battery voltage is measured for each voltage detection unit (battery block) so as to maintain the reliability of the battery pack. However, the battery characteristics, such as unevenness in internal resistance is not taken into consideration. In such voltage detection for each voltage detection unit (battery block) which does not take the battery characteristics into consideration, it is not possible to detect abnormal battery blocks with precision, thereby deteriorating the reliability of the battery pack.
According to one aspect of the present invention, there is provided a battery control device for controlling charging and discharging operations performed on a battery pack in which a plurality of serially-connected battery blocks each include a plurality of battery cells, and the device includes: a voltage measurement section for measuring a battery voltage value for each voltage detection unit determined according to a prescribed condition based on at least one battery characteristic; and an abnormal battery detection section for detecting an abnormal battery based on the battery voltage value measured by the voltage measurement section.
In one embodiment of the invention, the prescribed condition based on the at least one battery characteristic is at least any one of unevenness in internal resistance, precision in battery voltage measured by the voltage measurement section, and the number of battery cells sharing an internal space in each battery case.
In another embodiment of the invention, the number N of the battery cells in each voltage detection unit is determined according to any one of the following conditions (1) through (6):
(1) Max (|I1|, |I2|)xc3x972xcex94R0xc3x97N less than Valm,
where I1 denotes current input to a battery cell (I1xe2x89xa00), I2 denotes current output from the battery cell (I2xe2x89xa00), N denotes a positive integer, a range of abnormal detection current of a battery is I1 to I2 (|I1|xe2x89xa7|I2|) (ampere), unevenness in internal resistance of a battery cell is xc2x1xcex94R0 (ohm), and a voltage value per battery cell determined by battery characteristics is Valm (volt);
(2) I2 denotes discharge current and Valm denotes electric potential at which hydrogen is generated from a battery electrolyte according to condition (1);
(3) both I1 and I2 denote current values in the vicinity of zero, and Valm denotes an open-circuit voltage of the battery cell;
(4) a voltage detection unit N obtained according to condition (1) is multiplied by the precision, xc2x1xcex94Vacy (%), of a voltage detection unit system, such that
Max (|I1|, |I2|)xc3x972xcex94R0xc3x972 xcex94Vacyxc3x97N less than Valm;
(5) Max (|I1|, |I2|)xc3x972xcex94R0xc3x97N less than Ralmxc3x97Max (|I1|, |I2|);
where I1 denotes current input to a battery (I1xe2x89xa00), I2 denotes current output from a battery (I2xe2x89xa00), N denotes a positive integer, a range of abnormal detection current of a battery is I1 to I2 (|I1|xe2x89xa7|I2|) (ampere), unevenness in internal resistance of a battery cell is xc2x1xcex94R0 (ohm), and internal resistance per battery cell determined by battery characteristics is Ralm (ohm); and
(6) in a battery pack including battery blocks having space-sharing structure between battery cells,
N0xe2x89xa6Nxe2x89xa6Axc3x97N0,
where the number of space-sharing battery cells is A (A is a positive integer), and N determined according to the conditions (1)-(5) is N0.
Thus, the invention described herein makes possible the advantages of providing a battery control device for detecting an abnormal battery using a battery voltage measured for each voltage detection unit, which is optimized according to battery characteristics, so as to reduce the possibility of mistakenly detecting an abnormal battery, thereby improving the reliability of a battery pack.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.